The present invention is directed to an RGB laser light source and, in particular, to such a source that provides color balance over a wide temperature range.
Pico or compact projectors become more and more popular. There are two competing technologies to generate light in these projectors. One of them is using light-emitting diodes (LED) and another—lasers. While LEDs offer low cost, overall efficiency of the LED-based projectors is rather low because of the difficulties to collect light that is emitted by LED in a very broad angle. Projectors based on lasers offer considerably higher overall efficiency and much wider color range producing vivid life-like images with saturated colors.
Laser-based pico projectors utilize two architectures: scanning minor (low brightness of up to 20 lumens, limited by the eye safety regulations) and field sequential display panel (DLP or LCOS with much higher brightness of up to 280 lumens) [E. Buckley, Journal of the SID, V. 18, P. 944 and P. 1051 (2010)]. As most of display devices, laser projectors create images utilizing three primary colors: Red, Green, and Blue (RGB). Red (wavelength of about ˜640 nm) and Blue (˜450 nm) colors are produced by direct semiconductor laser diodes (LDs). Red LDs are based on gallium arsenide (GaAs) material and Blue LDs are based on gallium nitride (GaN). Samples of direct Green semiconductor laser diodes based on GaN with a wavelength of about 520 nm with low power of about ˜50 mW, just recently became available and used in the projectors based on scanning minor architecture. Composite Green lasers with wavelength of about 532 nm are based on the second harmonic generation in nonlinear crystal. Composite lasers that deliver much power of 500 mW and higher, are used in the panel-based projectors.
One of the most important parameters for any consumer electronics device is operating temperature range that defines range of ambient temperatures where specifications are met. This range varies depending on the design of the projectors. The LED-based projectors have a wider range in comparison with the laser-based ones. Such a difference is due to the fact that Red laser output parameters, such as wavelength and power, depend on its case temperature. The wide ambient temperature range of projector operation from 0° C. to 35° C. is illustrative. Temperature inside the projector is about 15° C. higher because of the heat dissipated by the lasers and driver electronics. Thus, operating temperature range of the laser case inside the package may be 35° C., from 15° C. to 50° C. As it happens, case temperature affects Red and Blue LD operation in a subtly different way. Both cases are considered below.
In case of Blue LD, wavelength varies with the case temperature almost linearly (assuming fixed output power specified for the particular LD) with the slope of about 0.04 nm/° C. The overall wavelength change is about 1.4 nm. Wavelength increases with the temperature and going to the higher sensitivity range of the human eye. However, change in the wavelength is rather small and basically does not affect color balance of the projected images. The operating current increases with the temperature to maintain output power, however, reduction of the voltage across LD keeps power conversion efficiency (PCE) almost the same across the whole range. Constant PCE means that heat dissipated by the Blue laser remains about the same across the whole range. Green LDs that are based on the same material GaN as the Blue ones behave similarly. However, specified output power of the Green LDs is rather limited and declines at elevated temperatures.
Output wavelength of Red LDs increases with temperature about five times faster than that for the Blue LD. An average slope is about 0.2 nm/° C. However, it could be as high as 0.4 nm/° C. at higher temperatures in high-power assemblies. As ambient temperature increases, the wavelength of the Red LD becomes longer. Human eye sensitivity in the Red range decreases rather fast as wavelength increases. It means that output power of the Red LD must be higher to maintain brightness and color balance. This requires higher LD current that leads to the higher heat dissipation inside the projector package, which contributes to further internal temperature increase in comparison with the ambient.
Data in Table 1 is calculated for a pico projector with 40 lumens of brightness utilizing a Red LD in the pulsed mode of operation. The main conclusion from the data in the Table 1 is that, at higher ambient temperatures, power consumption of the Red LD increases more than two times from about 730 mW to about 1760 mW. It is more than 1 W power consumption/dissipation increase. This is a very high power consumption jump for a compact device powered by the battery. Table 1 gives a somewhat optimistic estimate because additional temperature increase due to the higher power dissipation has not been taken into account. That additional temperature increase is estimated to be as high as 25%-30% of that for the operation at low temperatures. In this case, power consumption for the Red LD would increase up to about 2W. It would be almost three times higher than power consumption at low temperatures.
TABLE 1Power consumption of the Red LD depending on theambient temperature in the 40 lm pico projector.AverageAmbientLD Case(Pulsed)PowerTemper-Temper-Wave-OutputconsumptionatureaturelengthRed LDPower, mWby Red LD° C.° C.nmPCE %of Red LDmW01563535%255.25(1021)729203563923%296.25(1185)1288355064219%334.75(1339)1762
The wavelength of the Red LD can be stabilized to avoid thermal runaway in a portable device, while maintaining color balance either by using a thermal electric cooler (TEC) device and an external volume Bragg grating placed in the package or an internal grating integrated in Red LD chip structure. However, the external volume Bragg grating involves additional components, which would increase cost and volume of the package. The internal grating would add complexity in processing of LD at production process and increase the production cost.
On the other hand, the most efficient Green composite laser that was specifically designed for projection applications and used in many pico-projectors and prototypes already has TEC on board. It allows maintaining Green laser performance in a broad ambient temperature range from −30° C. to +60° C. Such laser may be provided as a compact optically-pumped solid-state laser designed for efficient nonlinear intra-cavity frequency conversion into desired wavelengths using periodically poled nonlinear crystals. Such device is disclosed in commonly assigned U.S. Pat. No. 7,742,510, the disclosure of which is hereby incorporated herein by reference. The laser is highly efficient and its miniature package has been specifically designed to fit in the pico projectors. The 808 nm and 880 nm IR pump LD in the Green composite laser is based on GaAs, the same material as in Red LD. It means that both IR and Red LDs can be handled and mounted using the same processes.